TM 55-1930-209-14&P-12
1.1.3
Continued
controlled by its inductance. As a result of this, the current through the winding rises linearly with time,
producing a constant, or flat-topped output voltage until core saturation takes place. At this time, the
current tries to increase to large values in an attempt to keep the rate of change of flux and hence the
output constant.
The current through Q7; however, is limited by the base drive provided by T1, which in turn depends upon
the output voltage. When the current and the resulting flux can no longer increase, the induced voltages
drop to zero and Q7 is cut off (open switch). Cessation of current in the upper half of the winding of T2
allows the flux to decrease toward its remanance value, inducing voltages of the opposite polarity in all
windings. This action holds Q7 off (open switch) and turns Q8 on (closed switch). The battery is now
connected across the lower half of the winding on T2, and the flux builds up linearly to saturation in the
opposite direction. The core thus oscillates between positive and negative saturation at a frequency
depending upon core area, number of turns on T2, and battery voltage. In the FA 390A Oscillator these
factors have been selected to produce a nominal free-running frequency of 200 HZ. The oscillator will run
at this frequency when unloaded, or terminated with a resistor.
Form No. 6310 0238
